ok so if Tim Schaefer really wants to make a sequel but the copyright is not expired
yet, seems like Lucas keeps it a secret about the expiration date (its been 10 years). It looks like Lucas whats Grim Fandango to be an epic game (no sequels) or something of the sort, then why they have only mentioned the game in their oficial site, no info on the game etc. And it looks that they are not interested in making adventure games so why are they keeping the rights?

Because they are a business and it makes no sense promoting a game made 10 years ago that was not even popular then!